1. Field of the Invention
This invention relates to aircraft heating systems, and in particular to a heat exchanger for aircraft exhaust.
2. Background of the Invention
Heating systems for aircraft frequently take exhaust heat from the exhaust pipes of an aircraft by means of a heat exchanger, and transfer the heat into the cabin of the aircraft via ambient air through ducts. Aircraft heating may be necessitated by the colder temperatures of higher altitudes, and also winter flight frequently mandates the use of aircraft cabin heat.
A typical aircraft heating system which uses exhaust gas heat from the aircraft engine generally involves a heat exchanger having a heat shroud around one or more exhaust pipes, whose heat shroud inlet takes in ambient air from the atmosphere. This ambient air is heated while it circulates through the heat exchanger, and then the heated air departs the heat shroud through a heat shroud exit, and from there flows into the aircraft cabin to heat the cabin. A valve is generally provided which regulates how much heated air is allowed to flow into the cabin, and thus controls the cabin temperature.
Exhaust heater shrouds must be sturdily built to withstand the vibrations of an aircraft exhaust, and also be reasonably air-tight, to prevent loss of heated air. It is also important to preserve the air-tightness of the exhaust pipes passing through the heater exhaust shroud. If an exhaust pipe were to leak carbon monoxide, carbon dioxide, and other exhaust oxygen-poor gasses into the exhaust shroud, these gasses could wind up in the cabin, and could cause asphyxiation of the occupant(s). One way to help preserve the integrity of exhaust pipes within the shroud is to keep welds to a minimum within the heat exchanger, and only use full-penetration fillet welds and stitch welds external to the exhaust pipes within the heat exchanger itself.
Another design objective in exhaust heater shrouds for aircraft is the maximization of contact between ambient air flowing through the shroud to be heated, and the hot exhaust pipes and collector. The more contact between fresh air flowing through the shroud to be heated and the hot exhaust pipes, the more efficient the exhaust heater shroud. One existing design provides J-pipes and straight pipes feeding into a collector, all encased in a shroud through which ambient air flows, but this design does not provide a serpentine path to maximize heating.
Thus, it would be desirable to provide a heat exchanger for aircraft exhaust which maximizes the contact between fresh air flowing through the shroud and the hot exhaust pipes. It would also be desirable to provide a heat exchanger for aircraft exhaust which keeps welds to a minimum within the shroud, and only uses full-penetration fillet welds and stitch welds external to the exhaust pipes within the shroud.